Optical current transformers/transducers (OCTs) produce digital and analog signals. In recent years, they have become more available, playing an increasingly important role in electric power grid upgrades. This requires developing accurate models for relay testing and relay performance evaluations to ensure that the system equipment is safe. This study presents several models developed for OCT: analog, digital, and complete models. It applies previously studied and presented frequency-response characteristics to develop a transfer function using a complex curve-fitting method. An analog circuit model is constructed from the developed transfer function via a network synthesis method. A digital model is presented by transforming the developed transfer function into a digital domain transfer function using bilinear transformation. The digital model implementation is performed by the direct form I block diagram. The optics model and electronics model of the device are developed separately and are then combined to present a complete model. The Jones calculus method is used for optical modeling, and a developed transfer function is used for electronics modeling. All of the developed models are verified through the frequency-response results and tested with PSpice and Matlab Simulink programs. The results show that the developed models can be used as legitimate OCT models.

Complete Model Development for an Optical Current Transformer

Modeling and simulation of polarization errors in Sagnac fiber optic current sensor

Sagnac fiber optic current sensor(S-FOCS) is a kind of precision instrument based on optical polarization and optical interference theory. There are several kinds of error characteristics from optical components in S-FOCS, including polarizer, integrated optical chip, quarter-wave retarder, sensing head and so on. With these errors, linearly polarized light wave in PM fiber and circularly polarized light wave in sensing fiber become elliptically polarized light wave, then, nonreciprocal phase shift induced by magnetic field of the current is interrupted by wrong polarization state. Focused on optical polarization error in S-FOCS, we analyze the characteristics of polarization errors associated with optical components in S-FOCS theoretically, and we demonstrate the methods to build optical polarization error models by using Poincare sphere and Jones matrix respectively. Then, based on model of polarization state in S-FOCS, we carry out simulation of polarization error characteristics in S- FOCS, and theoretically investigate the influence of several main error factors on optical polarization characteristics, including polarizer, phase delay in quarter-wave retarder, splice angular between quarter-wave retarder and PM fiber. Index Terms - Fiber optic current sensor, Fiber optic gyroscope, Optical polarization state, Phase retardation, Modeling.

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Optical Polarization Errors in Sagnac Fiber Optic Current Sensor

We present a new optical current sensor architecture, which is based on a polarimetric configuration and a control system for self-compensation of the Fara day effect taking place at the sensor he ad. After passing through a bulk Faraday sensor head, the light travels through the free space reaching a Faraday modulator placed some distance away from the conductor carrying the current. The firs t device acts a current transducer and the second one acts as a magneto-optical element operated in a closed-loop mode to compensate the angle of rotation of the polarization introduced by the sensor head. The control system operates in closed loop feedback through a simple current-driven solenoid, and this way, the optical output from the current sensor is maintained at a constant intensity. Considering that the optical and electrical parameters of the sensor head and the Faraday modulator are known, the electrical current applied to the solenoid can be measured, and thus the current flowing through the conductor can be calculated. Experimental results demonstrate the feasibility of the proposed device to measure remotely the current carried by the conductor. Key words: Optical current sensor, Faraday effect, closed loop feedback

Allen H. Rose

Papers

Optical Current Sensors for Electric Power Grid Modernization